These were the classes that implemented both the messages and the components of the messages. I certainly treated them as code within the application, I just didn't physically type the code for them.

The management part came in from the fact that my application was a simulator used to test another section of the overall network. I had to support 3 revisions of the messages, plus variations of the messages depending on what's been implemented on the section my app tested. So, at any given time, I might have 5-7 different variations of a given message in play at a given time. I probably invested 2-3KLOC in the management scripts, part of which was parsing the original spec for the messages (in XLS, if I remember correctly).